Devices based on optical waveguides with adjustable bragg gratings

ABSTRACT

A system for filtering light propagating in a waveguide is described. The system utilizes an adjustable periodic grating which induces mode coupling of predetermined frequencies of light propagating in the waveguide.

This application is a divisional (and claims the benefit of priorityunder 35 USC 120) of U.S. application Ser. No. 09/226,030, filed Jan. 6,1999. The disclosure of the prior application is considered part of (andis incorporated by reference in) the disclosure of this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to controlling light propagatingin a wave guide. More particularly, the present invention relates tousing gratings to cause mode coupling of light propagating in a waveguide.

2. Description of Related Art

Devices used in optical systems, such as in fiber optic communicationsystems and sensing systems, often benefit from the filtering or controlof light propagating in a wave guide. Examples of such devices include,but are not limited to, source lasers, optical amplifiers, filters andother integrated-optical components. One method of controlling andfiltering light utilizes diffraction gratings. Descriptions of suchdevices and how they benefit from diffraction gratings are described inT. Erdogan and V. Mizrahi, “Fiber Phase Gratings Reflect Advances inLightwave Technology,” February 1994 edition of Laser Focus World.

There are three techniques typically used to create a diffractiongrating in a wave guide to induce mode coupling or Bragg reflection. Themost common method uses ultraviolet light to induce a refractive indexchange in an optical fiber. A system for producing a periodic refractiveindex change in the optical fiber is illustrated in FIG. 1. In FIG. 1 afirst beam 104 of coherent ultraviolet “UV” light and a second beam 108of coherent UV light are directed at a photosensitive optical fiber 112.At the intersection of the first beam 104 and the second beam 108, aninterference pattern 116 is generated. The refractive index of thephotosensitive optical fiber 112 changes with the intensity of the UVexposure, thus an index grating with a periodicity determined by theinterference pattern 116 forms where the first coherent beam 104 and thesecond coherent beam 108 intersect.

A second technique for creating a grating in an optical fiber involvesetching a periodic pattern directly onto an optical fiber. In oneembodiment, a photomask is used to generate a periodic pattern in aphotolithographic process. An acid etch etches the grating or periodicpattern into the optical fiber. Such photomasks and etching are commonlyused in semiconductor processes.

A third technique to control light in a waveguide is used insemiconductor waveguides. In one embodiment, a layered growth is formedon the semiconductor wave guide to generate light reflection in the waveguide.

The described techniques for creating a grating on or in a wave guideare permanent. The gratings have a fixed periodicity at a fixed locationon the waveguide that cannot be easily changed. Thus, a particular waveguide and grating combination will have a predetermined transmissioncharacteristic. In order to change the characteristic, the entire waveguide segment containing the grating is typically replaced with a waveguide segment having a different transmission characteristic. Replacingwave guide segments is a cumbersome process requiring that each end beproperly coupled to the light source and the light receiving device.

Thus, an improved system and method to control light propagating in awave guide is needed.

SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus of controllinglight transmitted in a wave guide. The apparatus uses a holder to fix awave guide in a fixed position relative to an adjustable periodicgrating. The periodic grating is movable to at least two positions, inone position the periodic grating induces mode coupling in the waveguide, and in the second position the periodic grating does not inducemode coupling in the wave guide.

The present application also describes other devices. Examples ofvarious devices include:

1. A tunable apparatus for controlling light transmitted in a wave guidecomprising:

-   -   a waveguide holder; and    -   an adjustable periodic grating coupled to the waveguide holder,        the adjustable periodic grating moveable between a first        position and a second position.

2. The tunable apparatus of claim 1 wherein when the periodic grating isin the first position, the adjustable periodic grating produces modecoupling from a first mode to a second mode of at least one wavelengthof light propagating in the waveguide.

3. The tunable apparatus of claim 2 wherein moving the adjustableperiodic grating to a second position prevents mode coupling at the atleast one wavelength of light.

4. The tunable apparatus of claim 2 wherein in the first position, anair gap separates the adjustable periodic grating and the optic cable.

5. The tunable apparatus of claim 2 wherein in the first position, theadjustable periodic grating contacts the optic cable.

6. The tunable apparatus of claim 1 further comprising:

-   -   an index matching fluid between the waveguide and the grating.

7. The tunable apparatus of claim 1 wherein the distance separatinggrooves on the adjustable periodic grating is between 0.1 um and 10 mm.

8. The tunable apparatus of claim 2 wherein one wavelength of lightpropagating in a forward direction that is coupled into a backwarddirection is given by:λ=Λ/2n _(eff)wherein: Λ=periodicity of the grating, and

-   -   n_(eff)=the effective refractive index of the waveguide.

9. The tunable apparatus of claim 2 wherein one wavelength of lightpropagating in a core of the waveguide that mode couples into a claddingof the waveguide is determined by:λ=Λ (n _(core) −n _(cladding))wherein: Λ=periodicity of the grating

-   -   n_(core)=the refractive index of a core of the waveguide, and    -   n_(cladding)=index of refraction of the cladding of the        waveguide.

10. The tunable apparatus of claim 2 wherein one wavelength of lightcoupled from a first polarization mode to a second polarization mode isgiven by:λ=Λ (n _(s) −n _(f))wherein: Λ=periodicity of the grating

-   -   n_(s)=effective refractive index of the first polarization mode    -   n_(f)=effective index of refraction of the second polarization        mode.

11. The tunable apparatus of claim 2 wherein one wavelength of lightcoupled from a first transversal mode to a second transversal mode isgiven by:λ=Λ (n ₁ −n ₂)wherein: Λ=periodicity of the grating

-   -   n₁=effective refractive index of the first transversal mode    -   n_(f)=effective index of refraction of the second transversal        mode.

11(A). The tunable apparatus of claim 1 further comprising a secondperiodic grating coupled to the waveguide holder.

12. The tunable apparatus of claim 2 wherein the first periodic gratingproduce mode coupling of a first frequency, the tunable apparatusfurther comprising:

-   -   a second periodic grating coupled to the waveguide holder, the        second periodic grating producing mode coupling at a second        frequency, the adjustable periodic grating working in        cooperation with the second periodic grating to produce mode        coupling at a third frequency, the third frequency higher than        either the first frequency and the second frequency.

13. The tunable apparatus of claim 1 further comprising:

-   -   a screw coupled to the periodic grating to move the periodic        grating between the first position and the second position.

14. The tunable apparatus of claim 1 further comprising:

-   -   a spring coupled to the periodic grating to regulate a pressure        on the periodic grating and a waveguide in the waveguide holder.

15. The tunable apparatus of claim 1 further comprising:

-   -   a piezo-electric coupled to the periodic grating to move the        periodic grating between the first position and the second        position.

16. The tunable apparatus of claim 1 wherein the waveguide includes acladding and a cladding reduced region, the periodic grating in contactwith the waveguide in the cladding reduced region.

17. The tunable apparatus of claim 1 wherein the waveguide includes acladding and a cladding reduced region, the periodic grating orientedsuch that in the first position, the periodic grating intersects anevanescent field of a waveguide mode in the cladding reduced region.

18. The tunable apparatus of claim 1 where the periodic grating is achirped grating.

19. The tunable apparatus of claim 18 wherein a spacing of grooves inthe chirped grating varies quadratically.

20. The tunable apparatus of claim 18 wherein a spacing of grooves inthe chirped grating varies linearly.

21. The tunable apparatus of claim 1 wherein the periodic grating isrotatable to an angle such that an effective grating spacing to lightpropagating in the waveguide is a physical spacing of the periodicgrating divided by a cosine of the angle.

22. A method of filtering at least one frequency of light propagating ina waveguide comprising the acts of:

-   -   positioning a periodic grating to induce mode coupling of one        frequency of light in the waveguide;    -   determining the intensity of the one frequency of light        propagating in the waveguide; and    -   repositioning the periodic grating to change the intensity of        the mode coupling of the one frequency of light.

23. The tunable apparatus of claim 22 wherein the waveguide is anoptical filter.

24. The method of claim 22 wherein the act of repositioning furthercomprises:

-   -   rotating a screw coupled to the periodic grating to reposition        the periodic grating.

25. The method of claim 22 wherein the act of repositioning furthercomprises:

-   -   changing an electric potential applied to a piezo-electric to        move the periodic grating.

26. The method of claim 22 further comprising the act of:

-   -   side polishing the optical fiber before positioning the periodic        grating.

27. An optic equalizer comprising:

-   -   a waveguide to guide light;    -   a first adjustable periodic grating positioned at a first region        of the waveguide, the first adjustable periodic grating to        generate mode coupling at a first frequency;    -   a second adjustable periodic grating positioned at a second        region of the optical fiber, the second adjustable periodic        grating to generate mode coupling at a second frequency.

28. The optic equalizer of claim 27 wherein the first adjustableperiodic grating is coupled to a screw to move the first adjustableperiodic grating between a first position and a second position.

29. The optic equalizer of claim 27 wherein the first adjustableperiodic grating is coupled to a piezo-electric to move the firstadjustable periodic grating between a first position and a secondposition.

30. The optic equalizer of claim 27 further comprising:

-   -   a third adjustable periodic grating positioned at a third region        of the optical fiber, the third adjustable periodic grating to        generate mode coupling at a third frequency, the combination of        the first adjustable grating, the second adjustable grating and        the third adjustable grating to adjust the spectral content of        light propagating in the waveguide at a fourth position in the        waveguide.

31. A graphic equalizer comprising:

-   -   a waveguide;    -   a plurality of adjustable periodic gratings, each adjustable        periodic grating in the plurality of adjustable periodic        gratings independently adjustable to alter the intensity of a        corresponding frequency range of light propagating in the        waveguide.

32. A method of using a graphic equalizer comprising:

-   -   receiving light in a waveguide;    -   determining the desired spectral content of light at a position        on the waveguide;    -   adjusting at least one periodic grating to cause mode coupling        of an undesired frequency of light propagating in the waveguide.

33. A variable delay apparatus comprising:

-   -   a circulator to receive a light beam at an input port;    -   a variable delay line coupled to a processing port of the        circulator, the circulator to propagate the light beam from the        input port to the variable delay line;    -   a plurality of adjustable gratings, each adjustable grating        movable between a corresponding first position which produces        Bragg reflection in the variable delay line and a second        position which does not produce Bragg reflection in the variable        delay line, the circulator receiving Bragg reflected signals at        the processing port and outputting the Bragg reflected signals        at an output port of the circulator.

34. The apparatus of claim 33 further comprising:

-   -   a plurality of piezo-electrics, each piezo-electric in the        plurality of piezo-electrics coupled to a corresponding        adjustable grating to move the corresponding adjustable grating        between the first position and the second position.

35. The apparatus of claim 34 further comprising:

-   -   a voltage source;    -   a plurality of switches, each switch in the plurality of        switches to couple the voltage source to a corresponding        piezo-electric in the plurality of piezo-electrics such that        switching a switch induces a corresponding piezo-electric to        move a corresponding adjustable grating.

36. A variable delay apparatus comprising:

-   -   a waveguide segment; and    -   an adjustable periodic grating moveable along the waveguide        segment in the direction of light propagation in the waveguide,        the adjustable periodic grating inducing Bragg reflection in the        waveguide.

37. The variable delay apparatus of claim 36 further comprising:

-   -   a circulator to transmit light to the waveguide segment and to        receive light reflected within the waveguide segment.

38. The variable delay apparatus of claim 36 wherein the adjustableperiodic grating is moved by a screw.

39. A method of transfering a light signal from a first polarizationmaintaining fiber to a second polarization maintaining fiber comprising

-   -   coupling the first polarization maintaining fiber to the second        polarization maintaining fiber at a joint to form a combination        polarization maintaining fiber;    -   position a periodic grating to induce mode coupling of one        frequency of light in the combination polarization maintaining        fiber;    -   adjusting pressure between the periodic grating and the        combination polarization maintaining fiber to couple power        between a fast mode and a slow mode of the combination        polarization maintaining fiber.

40. A tunable polarizer comprising:

-   -   a polarization maintaining fiber including a core and a        cladding; and    -   a periodic grating coupled to the polarization maintaining fiber        to couple a first polarization mode of a first wavelength into        the cladding.

41. The tunable apparatus of claim 40 wherein the periodic gratingpresses against the polarization maintaining fiber to induce a periodicchange in a refractive index of the polarization maintaining fiber tocouple the first polarization mode of the first wavelength into thecladding.

42. A variable optical attenuator comprising

-   -   a first polarization maintaining fiber;    -   a periodic grating to re-orient a polarization of a signal        propagating in the first polarization maintaining fiber, and    -   a polarizer to attenuate a portion of the signal output by the        periodic grating.

43. The variable optical attenuator of claim 42 wherein the polarizerfurther comprising:

-   -   a second polarization maintaining fiber including a cladding and        a core to receive the output of the first polarization        maintaining fiber; and    -   a second periodic grating to couple a first polarization mode of        a selected wavelength propagating in the core into the cladding        of the second polarization maintaining fiber.

44. An add/drop filter comprising:

-   -   a periodic grating to reorient a polarization of a first        wavelength of light propagating in a polarization maintaining        fiber; and    -   a polarization beamsplitter having a first input port coupled to        the polarization maintaining fiber to direct the first        wavelength of light output from the polarization maintaining        fiber in a first direction and a second wavelength of light        output from the polarization maintaining fiber in a second        direction.

45. The add/drop filter of claim 44 wherein the polarizationbeamsplitter further comprises a second input port to couple a thirdwavelength of light into the second direction.

46. An optical filer comprising:

-   -   a bimodal fiber;    -   a periodic grating to switch a portion of a signal propagating        in the bimodal fiber from a first mode to a second mode; and    -   a bimodal coupler to separate the portion of the signal        propagating in the second mode from the portion of the signal        remaining in the first mode.

47. An add/drop filter comprising:

-   -   a multimode fiber supporting at least two transversal modes;    -   a transversal mode converter to convert predetermined        wavelengths of a first set of signals in the multimode fiber        from a first mode to a second mode; and    -   a bimodal coupler having a first input port coupled to the        multimode fiber to direct the predetermined wavelengths of the        first set of signals in a first direction and to direct other        wavelengths of the first set of signals in a second direction.

48. The add/drop filter of claim 47 wherein the transversal modeconverter further comprises:

-   -   a periodic grating coupled to a waveguide carrying the set of        signals to induce a periodic change in an index of refraction of        the waveguide to convert the predetermined wavelengths of the        set of signals from the first mode to the second mode.

49. The add/drop filter of claim 47 wherein the bimodal coupler furthercomprising a second input port to couple a second set of signals intothe second direction.

50. An optical buffer comprising:

-   -   a waveguide loop;    -   a bimodal coupler to output signals propagating in a first mode        from the waveguide loop, the bimodal coupler maintaining signals        in a second mode in the waveguide loop; and    -   a switchable mode converter including a grating to switch the        signal from the second mode to the first mode.

51. A tunable apparatus for controlling light transmitted in a waveguidecomprising:

-   -   a waveguide holder;    -   a periodic grating; and    -   a coupling device to couple the waveguide holder to the periodic        grating, the periodic grating adjustable to change a spacing        between the waveguide holder and the periodic grating.

52. The tunable apparatus of claim 51 further comprising:

-   -   a waveguide in the waveguide holder wherein the coupling device        adjusts the periodic grating to induce a periodic change in an        index of refraction of the waveguide.

53. A tunable apparatus for controlling light transmitted in a waveguidecomprising:

-   -   means for adjusting a pressure of a periodic grating against a        waveguide.

54. The tunable apparatus of claim 53 wherein the adjusting of thepressure induces a periodic change in an index of refraction of thewaveguide.

55. The tunable apparatus of claim 53 wherein the means for adjustingpressure includes a piezoelectric to move the periodic grating.

56. The tunable apparatus of claim 53 wherein the means for adjustingpressure includes a screw to move the periodic grating.

57. A tunable apparatus for controlling light transmitted in a waveguidecomprising:

-   -   means for adjusting a spacing between a periodic grating and a        center point in a waveguide, the adjusting of the spacing to        induce mode coupling of a signal propagating in the waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of the present invention will become more readilyapparent to those ordinarily skilled in the art after reviewing thefollowing detailed description and accompanying drawings wherein:

FIG. 1 illustrates a prior art system for creating a periodic grating ina wave guide.

FIGS. 2A, 2B, illustrate two embodiments of the invention to cause modecoupling in a waveguide.

FIGS. 2C, 2D, 2E, 2F and 2G illustrate alternative groove patterns whichmay be used for the periodic grating.

FIGS. 3A and 3B are a graphs which plot light intensity as a function ofwavelength output by one embodiment of the present invention.

FIGS. 4A and 4B illustrate cross-sectional views of a tunable apparatusused in one embodiment of the present invention.

FIGS. 5A and 5B illustrate use of a piezo-electric to move the periodicgrating in one embodiment of the present invention.

FIGS. 6A and 6B illustrate an optical fiber for use as a waveguide inone embodiment of the present invention.

FIG. 7A illustrates an apparatus to rotate a periodic grating withrespect to the direction of light propagation in one embodiment of theinvention.

FIGS. 8A, 8B and 8C illustrate alternative embodiments of implementingan optical equalizer implemented using a plurality of adjustableperiodic gratings.

FIG. 9 is a graph illustrating an example of the output of an opticequalizer.

FIG. 10 illustrates using a plurality of adjustable periodic gratings tocreate a variable delay line.

FIGS. 11A and 11B illustrate a variable delay line crated using oneadjustable periodic grating.

FIGS. 12A, 12B, 12C, and FIG. 12D illustrate using a grating inducedpolarization mode converter to connect two polarization maintainingfibers.

FIG. 13A illustrates wavelength selective polarization mode conversion.

FIG. 13B illustrates one embodiment of an add/drop filter including awavelength selective polarization mode converter and a polarizationbeamsplitter.

FIG. 14A illustrates using a grating induced polarization mode converterwith a fiber polarizer to form a variable attenuator.

FIG. 14B illustrates making a variable attenuator with a grating inducedfiber polarizer and a grating induced polarization mode converter.

FIG. 14C illustrates making a modulator with a grating inducedpolarization mode converter and a polarization beamsplitter.

FIG. 15A and FIG. 15B illustrate a variable attenuator and a modulatormade from a grating induced transversal mode converter and a bimodalcoupler.

FIG. 16A and FIG. 16B illustrate the operation of a bimodal coupler.

FIG. 17 shows an add/drop filter made from a grating induced transversalmode converter and a bimodal coupler.

FIG. 18 illustrates an optical recirculating delay line including agrating induced transversal mode converter and a bimodal coupler.

DETAILED DESCRIPTION OF THE INVENTION

The following invention describes a method and apparatus of using anexternal grating to cause mode coupling in a wave guide. In thefollowing invention, a number of terms will be used which are hereindefined. A wave guide holder as used in this application is any devicewhich holds a wave guide such that relative position, distance, orpressure between the waveguide and a periodic grating can be adjusted.In one embodiment, the waveguide holder holds the waveguide in a fixedposition while the position of the external grating is adjusted. In analternative embodiment, the waveguide holder is adjusted to move thewaveguide to different positions with respect to the external grating.Examples of wave guide holders includes, but is not limited to, a block,a substrate of a semiconductor wave guide, the insulation surrounding awave guide, or other apparatus which can be used to grip or preventunwanted movement of the wave guide relative to a periodic grating.Furthermore, although the term periodic will be used throughout thisapplication, the term “periodic grating” will be defined to includechirped gratings in which the periodicity of the grating is not constantacross the surface of the grating. Finally, the term “mode coupling”will be defined to include the coupling of light in the fiber betweendifferent transversal modes (such as L₀₁ and L₁₁ modes), between counterpropagation modes such as the forward and backward propagating modes(e.g. light propagating in the forward and backward directions), betweena core mode and a cladding mode (e.g., light confined in the fiber coreor leaked into the cladding), and between polarization modes (e.g., inbirefringent fibers where the light signal polarized along the slow axisis coupled to the fast axis). The term “adjustable grating” will bedefined to include movable gratings as well as gratings which are fixedin position, but are coupled to a waveguide holder which may berepositioned to move a waveguide with respect to the fixed position ofthe grating.

In the following application, techniques for changing the distance orpressure between grating and waveguide will be described. The describedtechniques will include using screws, piezo-electrics, and springs.However, other devices may be used, such as magnets and electro-magneticactuators. Likewise, the specification will describe implementing theinvention with a fiber optic cable although other wave guides may beused. It is understood that the following detailed description willinclude these specifics to illustrate the preferred embodiments and alsoto enable one of ordinary skill in the art to implement the invention,however, these specifics should not be interpreted to limit theinvention to only the embodiments described herein as other embodimentswhich would be obvious to one of ordinary skill in the art are alsopossible.

In one embodiment of the invention, an external periodic grating ispositioned to effect an internal refractive index (or phase) gratingthat influences the light signal propagating inside the waveguide. Inthe illustrated example FIG. 2A, the wave guide is an optical fiber 204.However, it is to be understood that the wave guide does not have to bean optical fiber and may include semiconductor wave guides and othermedia for channeling light.

One method of using an external grating to create an internal indexgrating is to press the external grating against the waveguide, as shownin FIG. 2 a. When the external grating 212 is pressed against the fiber(waveguide) 204, an index grating grating will be generated in thewaveguide via the photoelastic effect. Such an effect has beensuccessfully used by the inventor to induce birefringence in fibers andhence to control the polarization states of light. (U.S. Pat. No.5,561,726 by X. Steve Yao hereby incorporated by reference).

In FIG. 2A, the optical fiber 204 is pressed between a holder, which inone embodiment includes a flat block 208 and a periodic grating 212. Inthe illustrated embodiment of FIG. 2A the periodic grating 212 is formedin a grooved block 216. Movement of the grooved block 216 against theoptical fiber 204 which is held in position by the holder or flat block208 causes a periodic refractive index change inside the fiber with aperiodicity defined by the external grating. This periodic index changein turn causes mode coupling inside the optical fiber 204. Changing thepressure of the periodic grating 212 against the optical fiber 204 whichis held in position by holder or flat block 208 causes certain wavelengths of light propagating in the optical fiber 204 in one propagatingmode to couple to a different mode. The different mode includes, but notlimits to a transversal mode, a cladding mode, a polarization mode, anda counter-propagation mode. The changing pressure allows “tuning”(adjusting of the light propagation characteristics) of the opticalfiber 204. In particular, changing the pressure alters the couplingstrength or the amount of coupling between the two modes.

In another embodiment of the invention, periodic grating 212 does nothave to be in actual contact with optical fiber 204 to cause modecoupling. It is sufficient that periodic grating 212 is positionedwithin an evanescent field of the light propagating in optical fiber204. The distance the evanescent field extends from the core of opticalfiber 204 is typically fairly small, on the order of sub microns andmicrons. Therefore, the cladding of the fiber needs to be reduced orremoved to allow positioning of grating 212 close to the core andprovide a sufficiently strong influence on the light signal. Movingperiodic grating 212 in and out of the evanescent field causes light inoptical fiber 204 to couple from one mode to another, and hence changethe spectrum, polarization, or signal strength of the light. Indexmatching fluids or gels may also be applied between the periodic grating212 and optical fiber 204 to enhance the mode coupling. Evanescentfields and the effective distance of the evanescent field from thesurface of a fiber are well understood in the art.

Depending on the position of periodic grating 212 relative to waveguide204, three effects may be achieved. At a first position, the periodicgrating is far enough away spatially from the optical fiber 204 that itdoes not cause a perturbation of the field of the signal propagating inthe optical fiber. At a second position the periodic grating is withinthe evanescent field produced by the signal in the fiber. As a result,the light signal inside the fiber is influenced by the grating 212 andcouples inside fiber from one mode to another. In a third position, theperiodic grating 212 contacts optical fiber 204. The pressure of theperiodic grating on optical fiber 204 also creates a periodic change inthe index of refraction, n, of the fiber via the photoelastic effect.Thus the light signal inside the fiber is influenced by both theperiodic grating 212 itself and by the pressure induced grating insidefiber, resulting in possibly stronger mode coupling, including Braggreflection of light propagating in the optical fiber. Therefore, theamount of mode coupling can also be controlled by the position of theperiodic grating 212.

As previously indicated, there are many types of mode coupling that canoccur in an optical fiber. The periodicity, or the grating spacing ofthe fiber, and the optical wavelength of the propagating light determinethe type of the mode coupling. For example, when λ is the wavelength oflight field in the fiber (or waveguide), n_(c) is the effective index ofrefraction of the fiber core, n_(c1) is the effective index of thecladding, Λ is the grating spacing, then the condition for couplingbetween a forward and backward propagating modes, or Bragg reflection,is:Λ=λ/2n _(c)   (1)

For coupling light from a fiber core to a fiber cladding, the gratingspacing is determined by:Λ=λ/(n _(c) −n _(c1))   (2)

For mode coupling in a birefringent fiber between the fast mode (thepolarization of light is along the fast axis of the fiber) and the slowmode (the polarization of light is along the slow axis of the fiber),the grating spacing is:Λ=λ/(n _(s) −n _(f))   (3)where n_(s) is the effective refractive index of the slow axis and n_(f)is the index of the fast axis.

For mode coupling in a multimode fiber between two transversal modes,the grating spacing is:Λ=λ/(n ₁ −n ₂)   (4)where n₁ is the effective refractive index of mode 1 and n₂ is the indexof mode 2. Conversely, when a grating period is given, the wavelength ofthe light signal that may influenced by the grating can be calculatedusing Eq. (1) to Eq. (4).

These principals of Bragg reflection and mode coupling are wellunderstood in the art and are described in Yariv's Optical Waves &Crystal, pages 405 to 503.

Because the amount of mode coupling can be controlled by controlling thepressure of the grating on the fiber or the position of the grating,tunable devices for controlling the light signal inside the fiber can berealized. FIG. 3 illustrates the result of a tunable wavelengthselective variable attenuator based on the signal coupling between thecore and cladding modes. Such a device is important in wavelengthdivision multiple (WDM) access systems where the strength of thedifferent wavelength channels need to be precisely controlled. In FIG.3, the transmission characteristic of light through the waveguide atpredetermined wavelengths is plotted. In the experiment, a 2 cm longexternal grating was pressed onto a standard communication fiber with aremoved buffer. The grating was designed to coupled the light signal ofaround 1310 nm inside the core of the fiber into the fiber cladding.Once the signal coupled into the cladding, it will be stronglyattenuated because the cladding has extremely high loss compared withthe core. In one example, the index difference between the core and thecladding is 0.25×10⁻², a suitable grating spacing may be 524 um asdetermined using Eq. (2). Each curve 312, 316, 320 represents the outputof the waveguide at a particular pressure of the periodic gratingagainst the waveguide. At high pressure, curve 312 of FIG. 3 shows thata significant portion of the light is coupled out and attenuated. Atintermediate pressure, substantially less light at a given frequency isattenuated, as illustrated by curve 316. At low pressure, curve 320shows that very little light is attenuated. Thus by altering thepressure of periodic grating 212 against optical fiber 204, the amountof light reflected can be adjusted or “tuned.” Therefore, a narrowbandwidth variable attenuator is created.

FIG. 2B illustrates an alternate embodiment of the invention whichallows altering the periodicity of the grating. In FIG. 2B, the holder224 which holds the optical fiber 220 such that the fiber does not moveaway from grooved block 228 is also grooved with a second set ofperiodic grating 232. Thus, both holder 224 and grooved block 228contain corresponding periodic gratings 232 and 236. By moving the firstperiodic grating 232 with respect to a second periodic grating 236 inthe direction shown by arrow 240, the effective periodicity of theperiodic grating combination can be adjusted. When an offset distance244 is zero, the first grating 232 and second grating 236 coincide orare aligned, the effective periodicity of the two gratings is equal tothe periodicity of the first grating 232. However, when offset distance244 is a maximum, where maximum is defined to be when offset distance244 is equal to one-half of the periodicity of a grating, the effectiveperiodicity seen by the optical wave propagating fiber 220 is twice theperiodicity of first periodic grating 232.

FIG. 2C, FIG. 2D, FIG. 2E and FIG. 2F illustrate different patternswhich may be used for periodic grating 212, 236, to 236. In FIG. 2C arectangular periodic grating is illustrated for use in the grooved block216. FIG. 2D illustrates the use of a trapezoidal 254 periodic gratingin a grooved block 258 as illustrates in FIG. 2D. FIG. 2E illustrates asinusoidal periodic grating 262 in a grooved block 266. FIG. 2Fillustrates a triangular periodic grating 270 in a grooved block 274.The grooved blocks illustrated in FIG. 2C, FIG. 2D, FIG. 2E, and FIG. 2Fillustrate examples of groove patterns which may be used in the devicesshown in FIG. 2A and FIG. 2B. The periodic gratings illustrated in FIGS.2C through 2F are for example only, other periodic structures may beused as is understood by those of ordinary skill in the art.

FIG. 2G illustrates a chirped periodic grating 278 in a grooved block282. Although chirped grating 278 does not have uniformly spaced groovepeaks, for purposes of this application, chirped grating 278 is definedto be one type of periodic gratings. Chirped grating 278 includes aseries of grooves 286, 290, 294, the spacing of the grooves with respectto adjacent grooves can be defined by a mathematical function of aposition along the grating block 282. In one embodiment of the chirpedgrating, the spacing of grooves 286, 290 and 294 increases linearlyacross chirped grating 278. In an alternate embodiment, spacing ofgrooves 286, 290, 294 increases in a quadratic function across chirpedgrating 278. By altering the grating spacing across chirped gratingblock 282, the spectrum of the the light signal which undergoes modecoupling within the optical fiber 204 can be increased, therebyincreaseing the bandwidth of light affected by the grating. In the caseof Bragg reflection, a chirped grating can also be used to compensatedispersion of the light signal (F. Ouellette, “All fiber filter forefficient dispersion compensation,” Optics Letters, Vol. 16, No. 5, pp.303-305) and hence increase the fibers transmission rate.

FIG. 4A and FIG. 4B show cross-sectional views of an apparatus 400 usedto press a periodic grating 404 against an optical fiber 408. In FIG.4A, grooves cut into a grooved block 412 form the periodic grating 404.A holder 416 which in the embodiment shown in FIG. 4A includes a flatsurface 420 that supports an opposite side of the optical fiber 408. Theholder 416 and the grooved block 412 containing the period grating 404interact to clamp the fiber between the grooved block 412 and flatsurface 420.

A spring 424 presses the grooved block 412 to keep a constant pressureon the optical fiber 408. Spring 424 typically has a spring constant Ksuch that the force applied to the grooved blocks is equal to F=K·Xwhere X is the distance by which the spring is compressed.

The pressure applied by the spring is adjusted by changing thecompression of spring 424. In one embodiment, the pressure on spring 424is controlled by a screw 428. Threads 432 on the screw interlock withthreads 436 in the holder 420 such that rotation of screw 428 moves thescrew in and out of holder 420. Rotation of screw 428 such thatadditional pressure is applied to spring 424 causes grooved block 412 topress harder against optical fiber 408 resulting in a greater change inthe index of refraction of fiber 408 and more intense mode coupling ofthe predetermined wavelength of light. In one embodiment of theinvention, when screw 424 is rotated outward, a tip 440 of the screwattaches to a portion of the grooved block 412 lifting the grooved blockaway from the optical fiber 408 to prevent mode coupling of lighttransmitted in optical fiber 408.

Other methods for moving the periodic grating towards and away from theoptical fiber 408 may be implemented. For example, FIG. 5A and FIG. 5Billustrate using a piezo electric acurator to move the grooved block 412towards and away from optical fiber 408. FIG. 5A illustrates a piezoelectric stack 508. A power source such as voltage source 504 isconnected to a stack 508 of piezo electric elements 512, 516, 520.Altering the voltage applied across the stack 508 changes thedisplacement of stack 508. When the piezo electric stack 508 issubstituted for spring 424 of FIG. 4A, the grooved block 412 can bemoved towards or away from the optical fiber 408 by adjusting voltagesource 504.

Two methods of moving the grooved block have been illustrated in FIG. 4Band FIG. 5B. The first method uses a mechanical spring and screwarrangement structure while a second method uses a piezo electricdevice. Other methods of moving a grooved block are available to one ofordinary skill in the art. These methods may include but are not limitedto lever arrangements, and other mechanical, electro-mechanical,magnetic, or electro-magnetic devices suitable for moving an object oversmall distances. In the remaining description, various embodiments ofthis invention will be described using primarily a spring and screw,although it is understood that piezo electric stacks may be substitutedfor the screw spring arrangements as well as other mechanical andelectro-mechanical devices.

In one embodiment of the invention, the waveguide used is an opticalfiber which includes an optical fiber core surrounded by a cladding.When a clad fiber is used, a portion of the cladding may be reduced asillustrated in FIGS. 6A and 6B to improve the effectiveness of theperiodic grating. FIG. 6A illustrates a side polished fiber where aportion of the cladding 604 has been polished away to create a flatsurface 608. A periodic grating positioned against the side polishedflat surface 608 is in close proximity to the fiber core 612 such that alight signal propagating down the fiber core 612 is strongly influencedby the grating and undergoes mode coupling.

FIG. 6B illustrates one method of side polishing a clad optical fiber.In FIG. 6B, fiber 408 is placed in a substrate 616 which holds the fibersteady. The fiber 408 is then polished to create a flat surface 608approximately level with a top surface 620 of the substrate 616. In oneembodiment of the invention, the substrate 616 can subsequently be usedas the fiber holder to hold the fiber steady while the periodic gratingis applied to flat surface 608 of fiber 408.

The preceding description describes a basic tunable apparatus in whichthe periodic grating is moved in a direction perpendicular to thedirection of light propagating down a waveguide such as an optic cable.By adjusting the orientation or periodicity of a grating in the tunableapparatus or by repositioning the tunable apparatus, various devices canbe made.

In one embodiment of the invention, the periodic grating is rotatable.Rotating the periodicity of the grating changes the effectiveperiodicity of the gratings as illustrated in FIG. 7A. In FIG. 7Aperiodic grating 704 is rotated with respect to the direction of lightpropagation in waveguide 708 by an angle θ. Screw mechanism 712 is usedto apply pressure to one end of the periodic grating which rotatesperiodic grating 716. It is recognized that a piezo electric or otherdevice can be substituted for screw 712. The effective grating spacingof the rotated grating is equal to the spacing of the grating divided bythe cosine of the angle θ:Λ′=Λ/cos θ  (5)Changes in an effective grating also changes the wavelength of the lightsignal that undergoes mode coupling according to equations (1) to (4).When the grating spacing is selected such that light inside a fiber coreeither undergoes the Bragg reflection as defined in Eq. (1) or iscoupled out into fiber cladding as defined by equation (2) andillustrated in FIG. 3, a wavelength tunable variable attenuator (WTVA)is created. By adjusting both the angle θ and the position (or pressure)of the grating against the fiber, one is able to selectively attenuate asignal of any wavelength by a variable amount. Such a wavelength tunablevariable attenuator is extremely useful for wavelength division multiple(WDM) access systems to equalize optical powers in different channels.

Several such wavelength tunable variable attenuators cascaded togethercan operate as an optical spectrum equalizer. FIGS. 8A, 8B and 8Cillustrate various embodiments of such an optical equalizer using aplurality of tunable gratings. In FIG. 8A, a fiber holder 808 or casingholds a waveguide 804 steady. A plurality of moving devices such asscrews 812, 816, 820, 824 moves a corresponding grooved block. Across-section of one embodiment of an optical equalizer is illustratedin FIG. 8B. Each screw and spring arrangement corresponds to acorresponding grooved block. For example, screw 812 corresponds togrooved block 828, screw 816 corresponds to grooved block 832, screw 820corresponds to grooved block 836, and screw 824 corresponds to groovedblock 840. Each grooved block has a grating with a differentperiodicity. A user selects what frequencies or wavelengths of light tofilter and then selects a grooved block with a periodicity which willinduce mode coupling (including core/cladding coupling and Braggreflection) at the selected wavelength. The user then adjusts theselected grooved block to press against waveguide 804 and induce modecoupling at the desired wavelengths. By selecting and positioningperiodic gratings with predetermined periods against fiber 804, a userselects which wavelengths of light to couple out and thus filter.

FIG. 8C illustrates the optic equalizer illustrated in FIG. 8B where thescrews 812, 816, 820, 824 and corresponding springs have been replacedwith a plurality of voltage sources 844, 848, 852, 856 and correspondingpiezo electric stacks 860, 864, 868, 872 of piezo electric elements. Byadjusting the voltage of the voltage sources and thereby changing thedimensions of the corresponding piezo electric stack, each periodicgrating with its corresponding periodicity can be moved away from ortowards fiber 804 to induce mode coupling of light at the desiredfrequencies.

FIG. 9 is a graph illustrating an example output of an optical equalizerfor flattening the output spectrum of a WDM system. FIG. 9 plots theintensity of light on a Y-axis 904 with respect to the wavelength oflight which is plotted on X-axis 908. In the example, the “output” ofthe optical equalizer is defined to be the light which undergoes Braggreflection or other types of mode coupling. In the example, three peaks912, 916, 920 are the spectral “bumps” of the original signal input tothe equalizer. At the output, each peak is removed by a correspondinggrating with a properly adjusted angle θ and pressure (or position) viaa proper type of mode coupling. The removal of the highest peak 912requires more mode coupling, corresponding to higher pressure of agrating against a fiber in core/cladding coupling (or closer positioningof a grating against a fiber in the case of Bragg reflection). Removalof lower peaks requires a weaker mode coupling and therefore lesspressure.

FIG. 10 illustrates using a plurality of tunable apparatuses, eachapparatus including adjustable periodic gratings to create a variabledelay line via grating induced Bragg reflection. In the variable delayline of FIG. 10, an input 1008 of a circulator 1004 receives an incomingsignal. Light entering input 1008 exits the circulator 1004 at aninput-output port 1012 into a delay unit 1016. Delay unit 1016 includesa plurality of periodic gratings 1018, 1020, 1022, 1024. In oneembodiment of the invention, each periodic grating 1018, 1020, 1022,1024 has the same periodicity. A switch such as switches 1026, 1028,1030, 1034 couples a voltage source 1140 to the corresponding periodicgrating 1018, 1020, 1022, 1024. Closing a switch, such as switch 1026,moves a corresponding periodic grating 1018 towards a section of thewaveguide in delay unit 1016.

By selecting one grating in the plurality of gratings 1018, 1020, 1022,1024 to move against the section of waveguide 1044, a variable delay iscreated. Moving a periodic grating positioned far away from circulator1012 against the section of waveguide 1044 results in a long delaybecause light must travel from the circulator to the grating and thenreturn to the circulator 1012 before being output. Moving a periodicgrating such as grating 1018 positioned close to the circulator 1012,against the section of waveguide results in shorter delays because thelight has to travel only a short distance before being reflected back tothe circulator. The delayed signal re-enters circulator 1004 throughinput-output port 1012 and exits the circulator from output port 1052.In typical use of the invention, only one switch in the plurality ofswitches is closed creating one reflected signal with a predetermineddelayed time.

When the range of delays is not large, a simple delay circuit asillustrated in FIGS. 11A and 11B may be used. In the illustratedembodiment, a circulator such as circulator 1104 may be used. An inputport 1108 of circulator 1104 receives an input signal. A delay unit 1116processes the signal between the time the signal is input and output bythe circulator 1130. Delay unit 1116 includes a periodic grating whichis adjustable in a lateral direction 1120. A screw 1124 moves theperiodic grating 1122 in lateral direction 1120 to create variabledelays in the delay line segment 1112. Moving the periodic grating 1122in a lateral direction 1120 closer to the circulator results in shorterdelays while moving the grating 1122 in a lateral direction 1120 awayfrom circulator 1104 results in longer delays. The delayed signal alongdelay unit 1112 returns to circulator 1104 through input-output port1108 and is output through output port 1130.

FIG. 12A illustrates using an adapter 1204 to connect two polarizationmaintaining (PM) fibers 1208 and 1218. The apparatus of FIG. 12A usesthe relationship defined by equation (3) to rotate the polarization inpolarization maintaining (PM) fibers. As described in U.S. Pat. No.5,561,726 entitled “Apparatus and method for connecting polarizationsensitive devices”, traditional methods of interconnecting two PM fibersinvolves precision alignment of the fiber axes. However, using thedevice shown in FIG. 12A, one can simplify the cumbersome fiber axisalignment procedure. In the example illustrated in FIG. 12B, apolarization state 1248 of light propagating in PM fiber 1208 at theconnector ferrule 1210 is aligned with a slow axis 1240 of PM fiber1208. However, as illustrated in FIG. 12C, slow axis 1244 of receivingfiber 1218 is not aligned with slow axis 1240 at an input to a secondconnector ferrule 1212.

For light in receiving fiber 1218 to polarize along the slow (or fast)axis, polarization mode converter 1216 presses an external grating witha grating spacing 1238 defined by Eq. (3) against fiber 1218 to causecoupling between the two polarization modes. Rotating screw 1288 adjuststhe pressure of the external grating against fiber 1218 until asubstantial portion of the power polarized along the fast axis iscoupled into the slow axis, or vise versa. Consequently, polarizationmode converter 1216 aligns polarization state 1250 with slow axis 1244of receiving PM fiber 1218. In alternate embodiments of the invention, amechanical splice or a fusion splice may be substituted for fiberconnectors 1268 and 1278.

Because polarization mode coupling is wavelength dependent, signals of aselected wavelength may be coupled into one polarization while signalsof a second wavelength remain in an original polarization state. FIG.13A illustrates using polarization mode coupling to fabricate awavelength division multiplexer (WDM). In FIG. 13A, a first signal withwavelength λ₁ and a second signal with wavelength λ₂ propagate in PMfiber 1388. Both signals have polarization states 1300 and 1302 orientedalong the slow axis of PM fiber 1388. When a grating 1328 with a spacing1338 defined byΛ₁=λ₁/(n _(s) −n _(f))   (6)is applied against PM fiber 1388 with sufficient pressure, the firstsignal with wavelength λ₁ is coupled into a fast axis while the secondsignal with wavelength λ₂ remains oriented along the slow axis. Bycombining the wavelength selective polarization mode converter 1216 witha polarization beamsplitter (PBS) 1310, a wavelength divisiondemultiplexer is created to separate the signals of two wavelengths.

FIG. 13B illustrates using polarization mode coupling to fabricate anadd/drop filter. In FIG. 13B, the signal with wavelength λ₁ is outputfrom port 1318 of PBS 1310, while the signal with a wavelength λ₂continues through port 1316 of PBS 1310. In addition, a third signalwith a wavelength λ′₁ entering port 1320 of PBS 1310 is added to λ₂. oneaspect of the devices illustrated in FIG. 13A and FIG. 13B is that theWDM and add/drop filter is switchable, a feature that is extremelyuseful in WDM networks.

Gratings with selected grating spacing corresponding to a set ofselected wavelengths may be used to reorient the polarization of theselected wavelengths from a first polarization mode to a secondpolarization mode. Combining a set of polarization mode converters witha polarization beamsplitter allows separation of the selected wavelengthchannels from unselected channels. Such wavelength selectivepolarization mode conversion can be used to double the channel spacingof a WDM system. Wavelength selective polarization mode conversion mayalso be used in a fiber gyro system.

Core/cladding mode coupling in a PM fiber can also be used to fabricatea polarizer. Because the indices of refraction are different for a modepolarized along the slow axis and a second mode polarized along the fastaxis, core/cladding coupling may occur for one polarization mode and nota second mode. For example, when a grating spacing is chosen such thatΛ=λ/(n _(cs) −n _(c1)),   (7)where n_(cs) is the effective index of the guided mode polarized alongthe slow axis, n_(c1) is the effective index of the cladding, and λ isthe wavelength of the propagating light, the mode polarized along theslow axis will be coupled into the fiber cladding and be attenuated. Thesignal polarized along the fast axis will remain in the core andunaffected. Therefore, a polarizer is created without the light exitingthe fiber. By adjusting pressure of the grating against the fiber, theextinction ratio of the polarizer can be controlled to produce apolarization dependent variable attenuator.

A fiber switch or a variable attenuator may be formed by combining agrating induced polarization mode converter with a fiber polarizer. FIG.14A illustrates a polarization mode converter 1416 connected to a fiberpolarizer 1458. Adjusting the pressure of a grating against a section offiber rotates the polarization of a signal propagating in the fiber. Inthe illustrated embodiment, polarizer 1458 allows only one polarizationto pass, thereby reducing the power of the signal propagating in thefiber. A fiber optic modulator/switch can be realized by replacingadjustment screw 1488 with a piezo-electric actuator 1498 controlled byan electrical source 1486, as illustrated in FIG. 14C. FIG. 14Billustrates one embodiment of the invention in which polarizer 1458 ofFIG. 14A has been replaced with an all fiber polarizer 1450. All fiberpolarizer 1450 includes a grating with a spacing 1468 defined by Eq. (7)coupled to a section of fiber 1488. In FIG. 14C, a polarizationbeamsplitter 1490 replaces polarizers 1458 and 1450 in FIGS. 14A and 14Bto make a variable polarization separator or a fiber opticmodulator/switch with two complimentary output ports.

A transversal mode converter conforming to the relationship described inEq. (4) is especially useful in an optical fiber which supports twotransversal modes (bimodal fiber). In practice, any single mode fibercan be used as a bimodal fiber when the wavelength of a light signal inthe fiber is below the cutoff wavelength of the fiber. In the followingdescription, several devices which can be made using the wavelengthtunability and coupling strength tunability of the invention describedin FIGS. 4, 5 and 7 will be described.

FIG. 15A and FIG. 15B illustrate a fiber optic modulator, a switch, anda variable attenuator formed by coupling the output of the transversalmode converter 1580 with a bimodal coupler 1520.

FIG. 16A illustrates a bimodal coupler which separates two modes 1608and 1610 in a bimodal fiber 1602 into different fibers 1604 and 1606.The bimodal coupler can also be used to transfer two signals 1612 and1614 from two different fibers 1616 and 1618 into two differenttransversal modes in a bimodal fiber 1620, as shown in FIG. 16B. Abiconic fused coupler technique or side polished coupler techniquecommonly used in manufacturing fiber optic couplers and WDMs asunderstood by those of skill in the art can be used to make a bimodalfiber coupler. Positioning two bimodal fibers or one bimodal and onesingle mode fiber together in close proximity, as shown in FIG. 16A,induces mode coupling between the two fibers. Different propagationconstants of the two modes results in different coupling strengths ofeach mode. For a properly selected coupling strength (determined by thedistance between the two fibers, the propagation constant of the mode,and coupling length), one mode will be completely (or near completely)coupled into the other fiber and the remaining mode will remain in theoriginal fiber.

FIG. 15A illustrates adjusting a pressure on grating 1528 with a groovespacing defined by Eq. (4) against bimodal fiber 1588 to control modecoupling. Changing the amount of mode coupling changes the output fromport 1522 and/or port 1524 of bimodal coupler 1520, resulting invariable attenuation of the output signal. FIG. 15B illustrates using anelectrical actuator 1598 to control the mode converter 1580 such thatthe variable attenuator operates as a switch or a modulator.

Because the transverse mode converter is highly wavelength selective,the devices illustrated in FIGS. 15A and 15B can also be used as awavelength division multiplexer/demultiplexer or add/drop filter. Asshown in FIG. 17, multiple signals of different wavelengths propagatingin a first mode of bimodal fiber 1788 exit bimodal coupler 1720 from afirst port 1724. Activating transversal mode converter 1780 forwavelength λ_(i) converts wavelength λ_(i) signals in a first mode to asecond mode. Signals in the second mode exit coupler 1720 from a secondoutput port 1722. In one embodiment of the invention, signals in secondoutput port 1722 are removed (or dropped) from the system. To add asignal with a wavelength λ′_(i), the added signal is input into secondinput port 1712 of bimodal coupler 1720. A second mode converter may beused to ensure that the added signal is in the second mode. The bimodalcoupler 1720 combines the added signal with other propagating signals inport 1724 which may be connected to a system bus (not shown).

FIG. 18 illustrates using the mode converter 1880 and bimodal coupler1820 combination as a recirculating optical delay line. An input opticalpulse 1808 in a first mode, “mode 1”, is coupled into a bimodal fiberloop 1888 via bimodal coupler 1820. The pulse remains in mode 1 andexits loop 1888 from port 1822 of coupler 1820 after propagating aroundthe loop once when mode converter 1880 is in an off state. However, whenmode converter 1880 is activated to convert the pulse from mode 1 into asecond mode, mode 2, the pulse does not exit the loop and insteadpropagates around the loop until the mode converter is activated againto convert the pulse back to the first mode whereupon the optical pulseexits the loop from port 1822 of coupler 1820. By controlling the modeconverter as illustrated, an optical pulse may be delayed for acontrolled period of time. Such recirculating optical delay lines areuseful as a memory buffer in optical networks and in optical computers.

A similar recirculating optical delay line can also be made by replacingtransversal mode converter 1880 of FIG. 18 with a polarization modeconverter and replacing the bimodal coupler with a polarizationbeamsplitter.

While the Applicant has described various embodiments of the tunableapparatus which involves moving a periodic grating towards and away froma waveguide to induce mode coupling at certain predetermined wavelengthsand various devices which can be built from such a tunable apparatusincluding a variable delay line, an optical equalizer, a wavelengthdivision multiplexer, an add/drop filter, a polarization converter, anda wavelength selective variable attenuator, other embodiments and usesmay be apparent to one of ordinary skill in the art. Thus, the inventionshould not be limited to merely the embodiments described in thepreceding specification. The limitations of the application arespecifically claimed in the Claims which follow.

1. A variable delay apparatus comprising: a waveguide segment; and anadjustable periodic grating moveable along the waveguide segment in thedirection of light propagation in the waveguide, the adjustable periodicgrating inducing Bragg reflection in the waveguide.
 2. The variabledelay apparatus of claim 1 further comprising: a circulator to transmitlight to the waveguide segment and to receive light reflected within thewaveguide segment.
 3. The variable delay apparatus of claim 1 whereinthe adjustable periodic grating is moved by a screw.
 4. A method oftransferring a light signal from a first polarization maintaining fiberto a second polarization maintaining fiber comprising coupling the firstpolarization maintaining fiber to the second polarization maintainingfiber at a joint to form a combination polarization maintaining fiber;position a periodic grating to induce mode coupling of one frequency oflight in the combination polarization maintaining fiber; adjustingpressure between the periodic grating and the combination polarizationmaintaining fiber to couple power between a fast mode and a slow mode ofthe combination polarization maintaining fiber.
 5. A tunable polarizercomprising: a polarization maintaining fiber including a core and acladding; and a periodic grating coupled to the polarization maintainingfiber to couple a first polarization mode of a first wavelength into thecladding.
 6. The tunable apparatus of claim 5 wherein the periodicgrating presses against the polarization maintaining fiber to induce aperiodic change in a refractive index of the polarization maintainingfiber to couple the first polarization mode of the first wavelength intothe cladding.